Handle denied BLE permissions with user feedback

When BLUETOOTH_SCAN, BLUETOOTH_CONNECT or ACCESS_FINE_LOCATION
permissions are denied, show 'Permissies geweigerd — open Instellingen'
and disable the relay button instead of silently hanging.

Reviewed in codereview.md (recommendation 2.1).

android/app/src/main/java/com/eddy/atomrelay/MainActivity.kt

@@ -219,8 +219,13 @@ class MainActivity : AppCompatActivity() {
     @Deprecated("Deprecated in Java")
     override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<out String>, grantResults: IntArray) {
         super.onRequestPermissionsResult(requestCode, permissions, grantResults)
-        if (requestCode == 1 && grantResults.all { it == PackageManager.PERMISSION_GRANTED }) {
-            startScan()
+        if (requestCode == 1) {
+            if (grantResults.all { it == PackageManager.PERMISSION_GRANTED }) {
+                startScan()
+            } else {
+                statusText.text = "Permissies geweigerd — open Instellingen"
+                relayButton.isEnabled = false
+            }
         }
     }